ENUMERATION AND CELL-CYCLE ANALYSIS OF NATURAL-POPULATIONS OF MARINE PICOPLANKTON BY FLOW-CYTOMETRY USING THE NUCLEIC-ACID STAIN SYBR GREEN-I

The novel dye SYBR Green I binds specifically to nucleic acids and can be excited by blue light (488-nm wavelength). Cell concentrations of prokaryotes measured in marine samples with this dye on a low-cost com pact flow cytometer are comparable to those obtained with the W-excite d stain Hoechst 33342 (bis-benzimide) on an expensive flow cytometer w ith a water-cooled laser. In contrast to TOTO-1 and TO-PRO-1, SYBR Gre en I has the advantage of clearly discriminating both heterotrophic ba cteria and autotrophic Prochlorococcus cells, even in oligotrophic wat ers. As with TOTO-1 and TO-PRO-I, two groups of heterotrophic bacteria (B-I and B-II-like types) can be distinguished. Moreover, the resolut ion of DNA distribution obtained with SYBR Green I is similar to that obtained with Hoechst 33342 and permits the analysis of the cell cycle of photosynthetic prokaryotes over the whole water column.